This project involves longitudinal study of rhesus monkey biobehavioral ontogeny, emphasizing investigation of individual differences in behavioral and physiological reactions to mild environmental challenges and determination of long-term developmental consequences under standardized rearing conditions. Data collected this past year revealed complex but predictable relationships between genetic factors and particular postnatal experiences contributing to individual differences between same-aged monkey subjects. 1) Although nursery-reared and mother-reared juveniles achieved the same mean levels of performance on cognitive tests, the neonatal temperament characteristics of the top performers differed dramatically across the 2 rearing conditions. 2) Infants selected on the basis of genetic pedigree to be high vs. moderately reactive to challenge, then cross-fostered to multiparous females who differed in bothy their own reactivity and their characteristic maternal style, displayed species-normative development, with differences between infants best predicted by foster mother maternal style. However, when challenged in the absence of foster mothers differences between infants were best predicted by genetic pedigree, but when challenged in the presence of foster mothers, such differences were overridden by differences in foster mother reactivity. 3) Peer-reared infants selected on the basis of genetic pedigree showed predictable differences in behavior, HPA activity, and heartrate when introduced to an unfamiliar agemate in a playroom setting. 4) Major differences in imipramine metabolism were found when the antidepressant was administered to adolescent monkeys under baseline vs. social challenge conditions. 5) Suppression of immune response to mitogen challenge was found in low- and mid-ranking, but not high-ranking juvenile monkeys introduced to new social groups. 6) Measures of immunoglobulins in nursery-reared, mother-reared, and foster mother-reared monkey infants revealed major developmental changes, but no rearing condition differences. Finallly, 7) a pilot study of human mothers and infants obtained measures of heartrate and adrenocortical response, in addition to standardized behavioral ratings, under both control and mildly challenging circumstances.